Conventionally, there was a problem that streaked morphological abnormality occurred on surfaces of device substrates when the device substrates were fabricated by laminating device structures made of group 13 nitrides on Flux-GaN templates. FIG. 3 is a Nomarski differential interference microscope image of a surface of a device substrate on which the morphological abnormality occurs. In FIG. 3, the streaked morphological abnormality (line crack) is identified in a portion marked with an arrow. Since among device chips (LED chips, HEMT chips, etc.) obtained by dicing the device substrates, device chips formed in portions with the morphological abnormality may become characteristic defectives that cannot exhibit desired characteristics, occurrence of the morphological abnormality is a factor that reduces productivity of the device chips.
The factor causing the streaked morphological abnormality has not necessarily been clarified yet.
On the other hand, it is generally known that for example, laminating GaN films thicker on a sapphire substrate may cause a crack, and it has already been found that introduction of a low-temperature interlayer in a layered structure will be effective to prevent occurrence of such a crack (see, for example, Non-Patent Document 1).
However, there is a problem in this method that the resulting layered structure involves crystal defects due to use of a low-quality interlayer, and consequently, the crystal defects interfere with fabrication of high-quality devices (LEDs, HEMTs).